The order-disorder state of diaminoalkanes in Cu-based metal-organic materials

Order-disorder phase transition and molecular dynamics in the hybrid perovskite [(CH3)3NH][Mn(N3)3]

We present a temperature-dependent Raman scattering study of a [(CH₃)₃NH][Mn(N₃)₃] hybrid organic-inorganic azide-perovskite, in which we have analysed in detail the wavenumber and full width at half-maximum (FWHM) of lattice modes and internal modes of the NC₃ skeleton, N₃^- and CH₃ molecular groups. In general, the modes exhibited unusual behaviour during the phase transitions, including discontinuity in the phonon wavenumber, bandwidth, and unconventional shifts upon temperature variation. Spectral features on heating reveal the absence of significant distortions in the NC₃ skeleton and a relatively restricted order-disorder process of the TrMA⁺ cations. On the other hand, linewidth anomalies of the δNC₃ and ν_asNC₃ modes have been attributed to the molecular dynamics of encapsulated cations. The unconventional blue shift of the symmetric stretching modes of azide ligands indicates the weakening of the intermolecular interactions between the TrMA⁺ cations and azido-bridges, and the strengthening of the intramolecular bonds. Additionally, we have used differential scanning calorimetry to confirm the subtle monoclinic to monoclinic (P2₁/c → C2/c) phase transition at around 330 K; and the phase transition to trigonal structure (R3¯m) above 359 K, whose associated entropy variation turns to be |ΔS| ∼ 22.3 J·kg^-1 K^-1 and displays a barocaloric (BC) tunability |δTt/δP| ∼ 3.17 K kbar^-1, according to our estimations using the Clausius-Clapeyron method. Although the obtained values of entropy change and BC tunability are very close to those reported on formate-perovskites and other important caloric materials, those parameters are much lower than the giant entropy change of ∼80 Jkg^-1 K^-1 and large BC tunability ∼12 K kbar^-1 observed for the analogue azide-perovskite [(CH₃)₄N][Mn(N₃)]₃ (TMAMnN₃). Very interestingly, our combined study shed light to understand such different behaviour, as they reveal that the hydrogen bonds created between the TrMA⁺ cations and the framework prevent an extensive order-disorder process that is needed to obtain large entropy changes and large BC coefficients as it occurs in the case of related azide-perovskites with no H-bonds between the A cations (for example TMA) and the framework.

Dielectric switching in correlation with the structural phase transitions in tetrapropylammonium perchlorate

The crystals of the tetrapropylammonium perchlorate ([(CH₃CH₂CH₂)₄N]ClO₄, TePrAClO₄) compound undergo two reversible phase transitions: at ca. T₁ = 284 K and at ca. T₂ = 445 K. The observed phase transitions and distinct dielectric and relaxation effects are due to the dynamic motions of the organic cations and anionic framework. The crystals become ordered at low temperatures, then disordered at room temperature (propyl chains of the organic part as well as perchlorate ions are disordered over the mirror plane at c = 1/4 and 3/4) and highly disordered at high temperatures. The comparable changes in the wavenumber and FWHM shifts (IR and Raman spectroscopy) in the case of tetrapropylammonium and perchlorate ions in the phase transition at T₁ and slightly more significant changes for organic cations (juxtaposed with perchlorate ions) in the phase transition at T₂ lead to a conclusion that the phase transition at T₁ is equally driven by motions of the two ions, while the phase transition at T₂ is more influenced by the motions of organic cations. The phase transition at T₂ with its large entropy change resembles the behavior found in liquid crystals. The dielectric function values can be switched and tuned in the low- and high-dielectric states, which may indicate the potential application of this material in sensors or actuators.

Raman Spectroscopy Studies on the Barocaloric Hybrid Perovskite [(CH3)4N][Cd(N3)3]

Temperature-dependent Raman scattering and differential scanning calorimetry were applied to the study of the hybrid organic-inorganic azide-perovskite [(CH₃)₄N][Cd(N₃)₃], a compound with multiple structural phase transitions as a function of temperature. A significant entropy variation was observed associated to such phase transitions, |∆S| ~ 62.09 J·kg^-1 K^-1, together with both a positive high barocaloric (BC) coefficient |δT_t/δP| ~ 12.39 K kbar^-1 and an inverse barocaloric (BC) coefficient |δT_t/δP| ~ -6.52 kbar^-1, features that render this compound interesting for barocaloric applications. As for the obtained Raman spectra, they revealed that molecular vibrations associated to the NC_4, N₃^- and CH₃ molecular groups exhibit clear anomalies during the phase transitions, which include splits and discontinuity in the phonon wavenumber and lifetime. Furthermore, variation of the TMA⁺ and N₃^- modes with temperature revealed that while some modes follow the conventional red shift upon heating, others exhibit an unconventional blue shift, a result which was related to the weakening of the intermolecular interactions between the TMA (tetramethylammonium) cations and the azide ligands and the concomitant strengthening of the intramolecular bondings. Therefore, these studies show that Raman spectroscopy is a powerful tool to gain information about phase transitions, structures and intermolecular interactions between the A-cation and the framework, even in complex hybrid organic-inorganic perovskites with highly disordered phases.

The mechanism of phase transitions in azide perovskites probed by vibrational spectroscopy

The temperature-dependent IR and Raman spectroscopy has been used to study the phase transitions in manganese-azide frameworks with either dimethylammonium (DMA⁺) or tetramethylammonium (TMA⁺) cations which adopt a perovskite-like crystal structure. The phase transition in DMA-analogue seems to be associated with cooperative tilting of MnN₆ octahedra and order-disorder of hydrogen bonds while in TMA-analogue it is more complex and composed of several processes, including the motions of both manganese-azide framework and tetramethylammonium cations and their possible coupling. Our results are in agreement with the data received from crystallographic and dielectric measurements in the case of TMA-analogue and we have demonstrated the importance of order-disorder of hydrogen bonds in the case of DMA-analogue which previously has not been taken into account.

Polar metal–formate frameworks templated with 1,2-diaminoethane–water assemblies showing ferromagnetic and ferroelectric properties

A set of formate frameworks templated with 1,2-diaminoethane-water assemblies of the formula: [NH₃(CH₂)₂NH₃]M₂(HCOO)₆·H₂O (M = Mg, Mn, Co, Ni, Zn) has been synthesized. They show room-temperature ferroelectricity and ferromagnetic ordering below 35 K.